1. Technical Field
The instant disclosure relates to a DC-DC boost converter, in particular, to an audible noise avoiding circuit and a DC-DC boost converter having the same.
2. Description of Related Art
Please refer to FIG. 1. FIG. 1 is a circuit diagram of a conventional DC-DC boost converter. The conventional DC-DC boost converter includes an inductor 11, a first switch 12, a capacitor 13, a second switch 14, a control circuit 15, a voltage division unit 16 and driving units 17, 18. The control circuit 15 controls the DC-DC boost converter to operate in a pulse width modulation mode (PWM mode) or a pulse frequency modulation mode (PFM mode). The pulse frequency modulation mode is suitable for the light load condition. The control circuit 15 obtains a feedback voltage FB corresponding to an output voltage Vout via a feedback node 104 of the voltage division unit 16. According to the feedback voltage FB, the control circuit 15 controls the conducting status of the first switch 12 and the second switch 14 through the driving units 17, 18 respectively to adjust the voltage level of the output voltage Vout. When the voltage level of the output voltage Vout is insufficient, the second switch 14 and the first switch 12 are sequentially switched on to make the inductor 11 charge the capacitor 13. When the voltage level of the output voltage Vout is sufficient, the second switch 14 and the first switch 12 will not be switching.
Please refer to FIG. 1 and FIG. 2. FIG. 2 is a waveform diagram of voltage and current during operation of the DC-DC boost converter shown in FIG. 1. When the second switch 14 is turned on due to a voltage change at a control node G2 of the second switch 14, an inductive current rises with a slope Vin/L; L is an inductance value of the inductor 11. When the second switch 14 is turned off, and the first switch 12 is turned on due to a voltage change at a control node G1 of the first switch 12, the inductive current falls with a slope (Vin-Vout)/L. It is worth mentioning that when the second switch 14 is turned on, the voltage level at a phase node 103 drops from the input voltage Vin to a voltage level close to a ground voltage (zero volts). After that when the second switch 14 is turned off and the first switch 12 is turned on, the voltage level at the phase node 103 changes from a voltage level close to the ground voltage (zero volt) to the output voltage Vout.
The switching time interval T between each switch operation (or so called driver operation) varies due to the output loading at the output voltage node 102. Each switching of the first switch 12 and the second switch 14 could introduce audio noise. The frequency of the audio noise made by the first switch 12 and the second switch 14 depends on the switching time interval T.